Computer disc drives are dynamic information storage units having high bit densities. They are very high precision units requiring close dimensional tolerances in manufacturing and in use and are complex and delicate mechanically. They generally comprise rotatable memory discs, transducer heads and a linear or pivotally mounted, magnetically driven actuator arm assembly which supports the transducer heads and provides bidirectional movement with respect to the discs between inner and outer radial limits.
Limit stops are provided to control the limits of movement of the actuator arm assembly. By this expedient, the inner and outer radial limits of travel of the transducer heads with respect to the discs are established. The limits are required for safely restricting the travel in case of drive electronic failure and to establish known locations on the disc to provide information in recovery from a failure. In view of the high bit densities on the discs, it is important that the radial distance over the disc required for decelerating and stopping the heads be minimized. Equally importantly, the established radial limits must be precise as to location, precise as to the maintenance of that location, precise as to the maintenance of the stopping distance and must be accomplished in a manner that does not induce vibration or "ringing" of the delicate transducer heads. The ringing is due to the abrupt pickup of the crash stop mass (stationary) by the actuator assembly mass (moving).
The actuator arm assemblies are made of light weight material and the moving parts of the magnetic driver coupled to the actuator arm assembly are kept as light as possible, with as much stiffness as possible. This results in as high a natural frequency of vibration as possible. The impact of the actuator arm assembly with the limit stops may have high energy at high frequencies. This makes the actuator arm/transducer assembly vibrate a great deal in spite of the stiffness and frequency. Above a certain level, this vibration can cause damage to the delicate head assemblies, each of which comprises a transducer head mounted to a flexure assembly. The flexure comprises a load finger functioning as a cantilever spring, and a gimble mount for the transducer head. The transducer has two degrees of angular freedom, one in pitch and one in roll in its gimble mount.
Efforts to eliminate this problem in the past have resulted in the use of softer materials in the limit stop. These materials still have a mostly linear deflection or displacement rate. Normally the use of softer materials just increases the required stopping distance. This increase in radial stopping distance reduces the usable surface of the disc for information recording. In addition, such material as elastomers which have been used tend to vary considerably in stiffness as a function of temperature. When the design is made soft enough to function correctly at low environmental operating temperatures, by the time high environmental operating temperatures have been reached, along with additional temperature rises due to self heating of the drive system, the elastomeric material has been further reduced in stiffness. This not only increases the stopping distance it also changes the radial position in which the movement of the transducer heads ceases.
In some prior art arrangements employing a pivoted or rotary actuator arm, adjustment of the limit stop position has required the adjustment of limit stop members from one of the sides of the actuator housing in which the actuator arm assembly is pivoted. Usually these adjustments are quite sensitive requiring repeated attempts to achieve a precise limit stop setting. Additionally this increases the manufacturing costs since fabrication now requires off axis machining, that is drilling, reaming, boring, etc., in a direction other than a direction substantially paralleling the axis about which the actuator arm assembly is pivoted.
Other prior art designs have provided limit stop adjustments which are accessable from the top of the actuator housing. Some of these have a sliding part in which the slightest sliding movement to ajust the stop results in a significant change in the limit stop setting because of the small dimensions of track spacing. Such an arrangement while presenting a problem in achieving a desired limit stop position or setting, frequently also presents a problem in maintaining a fixed limit stop setting because of the high clamping forces which are required to secure the adjustable stop.